Recently, along with development of an optical communication system, middle/small-capacity transmission systems from a public communication system to an optical LAN (Local Area Network) system used in a private enterprise or the like have been increasingly put into practical use. Therefore, a field of optical measuring instrument is expected to be developed in consideration of the above situation.
A stabilized light source used in such a system is conventionally designed to satisfy optical output stability to cope with an application such as optical loss measurement or a fusing light source for an optical fiber fusion splice. However, a demand has arisen for an optical signal generator capable of varying an output power level and guaranteeing output level accuracy in order to respond to various needs as a light source.
On the other hand, optical power meters have been widely used along with recent rapid development of an optical industry, so that each user has a considerable number of optical power meters. In such a situation, a demand has arisen for maintaining accuracy of an optical power meter, i.e., accurately measuring a value of optical power with high reproducibility. Therefore, calibration of an optical power meter has attracted a great deal of attention.
Conventionally, calibration of an optical power meter is entrusted to a manufacturer of the meter, or if a traceability system is established in a company holding the meter, calibration is performed in a calibration department of the system. If calibration is entrusted to the manufacturer, however, a long time period and a transportation cost of an instrument are necessary for calibration, resulting in inconveniences. On the contrary, if calibration is performed using a standard in the company holding the meter, large scale equipment must be installed in order to maintain the standard. Currently, however, not all companies can have their own calibrating equipment.
An optical signal generating apparatus introduced in "The Anritsu Technical", No. 53, Mar. 1987, PP. 26 to 32 (issued by ANRITSU CO., LTD. of Japan) as an example of the above conventional optical signal generators adopts a circuit arrangement as shown in FIG. 1. That is, in FIG. 1, beam emitted from a light-emitting element, e.g., laser diode 1 is input to beam splitter 3 through continuous optical attenuating element 2. The beam passing through splitter 3 is attenuated by step optical attenuating element 4 to be a desired optical power level set value externally set by optical level setting circuit 9 to be described later. The output beam attenuated to be the predetermined value by element 4 is input to output fiber 5 and output from an output terminal, e.g., optical receptacle 6.
Meanwhile, beam split by splitter 3 is received by photodetector 7 and then converted into an electrical signal proportional to a received light level by O/E (Optical/Electrical converting) amplifier 21. The electrical signal is input as monitor output information to motor driver 11, and set information from D/A converter 10 is compared with the monitor output information by driver 11. As a result of comparison, driver 11 outputs a control signal to motor 12. Then, motor 12 for varying an attenuation amount of continuous optical attenuating element 2 operates to perform a power fine adjustment of the output beam from receptacle 6, and an optical power level equivalent to the set value set by setting circuit 9 is output.
Converter 13, driver 14 and motor 15 constitute a control circuit for changing the attenuation amount of step optical attenuating element 4 stepwise, and the set level value set by setting circuit 9 is displayed on display 20.
The beam emitted from laser diode 1 is received by monitoring photodetector element 16, and constant beam is always output by a stabilizing feedback loop consisting of element 16, amplifier 17, comparator 18, and driver 19.
Microprocessor (CPU) 8 connected to D/A converters 10 and 13, display 20, and setting circuit 9 through bus 0 performs predetermined control required for the entire system.
In such a conventional optical signal generating apparatus, however, an absolute value output level of the optical power after beam splitter 3 is not compensated for. Therefore, in the conventional apparatus, an error is produced in the optical power level of the output beam from receptacle 6 with respect to the set value set by setting circuit 9 due to a fiber loss at output fiber 5, contamination on the end face thereof, reflection on the end face, and factors associated with time such as an ambient temperature change. In addition, if the attenuation amount of element 4 changes due to contamination or the like, the optical power level of the output beam cannot be corrected because the change is not detected, resulting in output variations.
In this apparatus, a fine adjustment of the output optical beam is performed by continuous optical attenuating element 2 driven by motor 12. However, if resolution with high accuracy is required as in a calibrating system for the optical power meter, control finer than the fine adjustment performed by element 2 is sometimes desired.
FIGS. 2A, 2B and 3 are block diagrams showing a conventional system for calibrating the optical power meter. The conventional system will be described with reference to these drawings. In calibration of the optical power meter, instruments are connected in basically two manners as shown in FIGS. 2A and 2B.
A first system shown in FIG. 2A is the most basic one in which standard optical power meter 102 and optical power meter to be calibrated 103 are alternately connected to the output terminal of light source 101 through optical connectors 128 and 129a or 129b, thereby obtaining a calibrated value from a difference between indicated values of the both.
In this system, however, an optical signal output from light source 101 must be maintained constant to satisfy predetermined calibration precision while the two optical power meters are switched to perform calibration. In addition, if stability of the signal source is unsatisfactory, the above comparison must be repeated a plurality of times.
Therefore, calibrating the optical power meter using the optical signal generating apparatus as shown in FIG. 1 as light source 101 poses a serious problem of precision.
In a second system shown in FIG. 2B, the optical signal output from light source 101 is branched by optical branch circuit 104, and two branched optical signal outputs are detected by standard optical power meter 102 and optical power meter to be calibrated 103 through optical connectors 128a and 129a, and 128b and 129b, , respectively. At the same time, indicated values of both the optical power meters are obtained, thereby obtaining a calibrated value from a difference between indicated values of the both. In this case, a ratio (branch ratio) between the two optical signal outputs from optical branch circuit 104 must be accurately measured beforehand. That is, assuming that a ratio between standard output light P42 and calibrating output light P43 from circuit 104 is .gamma. and the indicated values of power meters 102 and 103 are P2 and P3, respectively, deviation factor .epsilon. of optical power meter to be calibrated 103 with respect to standard optical power meter 102 is given by the following equation: EQU .epsilon.=(p3/.gamma.p2)-1 (where .gamma.=P 43/P42)
However, in order to perform calibration with high precision as described above, both the first and second systems shown in FIGS. 2A and 2B have the following problems. That is, in the first system shown in FIG. 2A, the problem is light source 101, i.e., stability of the optical signal output during measurement. In the second system shown in FIG. 2B, the branch ratio of optical branch circuit 104 must be accurately measured in addition to the above problem. Especially when the second system in FIG. 2B is adopted to calibrate the optical power meter, it is difficult to stably maintain the branch ratio over a long time period because circuit 104 is arranged such that the branch ratio of the two outputs is adversely affected even by a very small change in element surface state. Therefore, in order to maintain a high calibration precision, branch ratio .gamma. must be frequently measured, resulting in frequent maintenance of the system.
In order to eliminate the above drawbacks of the two calibrating systems shown in FIGS. 2A and 2B, a third system shown in FIG. 3 is proposed. A shown in FIG. 3, the optical signal output from light source 101 is branched into monitoring output light 106 and calibrating output light 107 by optical branch circuit 104. Since variations in calibrating output light 107 can be monitored by detecting monitoring output light 106 branched by circuit 104, a calibrated value of optical stability of light 107 can be corrected if necessary.
In the system shown in FIG. 3, since monitoring output 106 is used as merely a reference signal, the branch ratio need only maintain a predetermined value only during a predetermined time interval in one measurement. Therefore, no absolute value of the branch ratio is required.
In calibration of the optical power meter, any of the following three calibration conditions is generally adopted.
(i) A deviation of an accurate value of a standard instrument with respect to a specific designated value of an instrument to be calibrated (most generally, a full scale value of each measurement range) is obtained. This is scale calibration based on the instrument to be calibrated. A so-called "scale calibration" means this condition and is widely adopted in calibration of an analog display meter.
(ii) A deviation of an indicated value of an instrument to be calibrated with respect to an accurate value of a standard instrument corresponding to a specific designated value of the instrument to be calibrated is obtained. This is scale calibration based on the standard instrument in which a deviation of each full scale value can be advantageously directly obtained when the instrument to be calibrated has an over range scale in each measurement range.
(iii) When a strictly specified value is not used and calibration is performed near a substantially designated value, a deviation of an instrument to be calibrated with respect to an accurate value of a standard instrument is obtained. This can be called calibration using a calibration coefficient and easily adopted when linearities of both the two optical power meters are guaranteed.
In the calibration conditions of (i) and (ii), an output must be accurately set to be a designated level with precision corresponding to indicating resolution of the standard optical power meter or predetermined calibration precision. However, it is often difficult to accurately set the output by only setting the level of the optical signal source especially in a measurement of an optical fiber system in which an optical fiber end level changes in accordance with the characteristics of a used optical fiber or the type of a used optical connector. Therefore, the optical signal output must be controlled in correspondence with the indicated value of the standard optical power meter. In addition, the set output level with respect to the standard optical power meter must be maintained with stability corresponding to the predetermined calibration precision until comparison of the two optical power meters is completed.
In order to perform calibration with high precision in the above conventional optical power meter calibrating system, the optical signal output within a measurement time interval output from the optical signal generating apparatus as a light source must be stabilized and the branch ratio of the optical branch circuit must be accurately measured. However, calibration cannot be performed with sufficient precision unless the level of the optical signal output is set and stabilized with satisfactory precision.